The Vaccine Advisory Committee approved a field test of Salk’s polio vaccine. The trial began the next day, with the vaccination of thousands of schoolchildren.
In all, over 1.3 million children participated in the trial. The trial was a randomized, double-blinded test, meaning that children were randomly assigned to either the control group or the vaccine group. Neither the children (nor their parents) nor health officials knew which children had received the vaccine and which had received the injected placebo fluid. (A smaller control group received no injection. Rather, officials observed them throughout the trial period for signs of polio infection.)
It would take almost a year to analyze the results and determine whether the vaccine provided protection against polio.
Source – historyofvacines